A lithium ion secondary battery is lighter in weight and has higher capacity than a nickel-cadmium battery, a nickel hydrogen battery, or the like. For this reason, the lithium ion secondary batteries have been widely used as a power supply for mobile electronic appliances. The lithium ion secondary battery is also a strong candidate for a power supply to be mounted on hybrid automobiles and electric automobiles. With the size reduction and higher sophistication of the mobile electronic appliances in recent years, the lithium ion secondary battery used for the power supply is expected to have higher capacity.
The capacity of a lithium ion secondary battery mainly depends on an active material of an electrode. As a negative electrode active material, graphite is typically used. To meet the above demand, however, it is necessary to use a negative electrode active material with higher capacity. In view of this, metal silicon (Si) with much higher theoretical capacity (4210 mAh/g) than that of graphite (372 mAh/g) has attracted attention.
An example of the negative electrode active material including such metal silicon is a mixture of silicon and silicon oxide. In the case of using the mixture of silicon and silicon oxide, the stress due to the expansion and contraction of silicon in the charging and discharging is relieved by silicon oxide. Thus, this mixture is superior to silicon in the cycle characteristics. On the other hand, the mixture of silicon and silicon oxide has low electric conductivity. Therefore, when the current density in the discharging is high relative to the battery capacity, the discharge capacity is remarkably deteriorated. In view of this, to achieve higher discharge capacity at a high rate has been an important issue to enable the battery to be used as a power source for hybrid vehicles and electric vehicles.
In the technique suggested to suppress the deterioration in discharge capacity at a high rate, the surface of the negative electrode active material is covered with carbon to allow the negative electrode active material layer to have higher electric conductivity and higher discharge capacity at a high rate.